Normal hydrocarbon well perforating operations require shutting down radio frequency (RF) transmitters and eliminating stray voltage sources before arming explosive equipment such as perforating guns at the surface of an oil or gas well. The exception is for certain qualified high voltage initiators as recommended by the American Petroleum Institute (API Recommended Practice 67 (RP67), 2nd Supplemental Edition, 2007) where explosive preparations are allowed in the presence of uncontrolled external voltages. High voltage initiators (HVI) include devices that utilize exploding foil initiation (EFI) and exploding bridge wire (EBW) as the initiating elements. An HVI that uses a semi-conductor bridge (SCB) is safer than a hot-wire detonator but more restrictive than HVIs using EFIs and EBWs.
These technologies were adapted for downhole during the last two decades. The first commercial EFI device for downhole use is described in U.S. Pat. No. 5,088,413 by Huber et al. The efficiency of such devices is determined in part by the overall inductance of a current loop that connects a capacitor, a switch and an EFI or EBW. One simple version was designed in the 1980s by Meyers, Application of Slapper Detonation Technology to the Design of Special Detonation Systems, Los Alamos Report LA-UR-87-391 that used a two conductor flexible cable that incorporated a small hole in the flex cable that served as a barrel between the EFI and the explosive pellet. The capacitor, switch. EFI and flex cable with a hole, used as an EFI infinite flyer barrel, were all part of the same current loop that reduced total resistance and inductance. This concept was followed in another design in the presentation of Lerche and Brooks, “Efficiencies of EFI Firing Systems,” 43rd.sup NDIA Fuze Conference, April, 1999.
The present high voltage devices for downhole explosive detonations are physically larger than conventional low voltage detonators (commonly called hot-wire detonators that utilize primary explosive), which normally have a slim profile. Low voltage detonators typically are about 0.3-inch diameter and less than 3 inches long. One advantage in using a low voltage detonator is afforded by its small size which allows its insertion into a perforating gun or firing head housing sub-assembly through a relatively small port plug, typically 13/16-inch or 1 and ⅜-inch diameter, permitting easy attachment outside the gun housing of the detonator to the wireline and then to the detonating cord, for example, before inserting the armed detonator back through the port plug hole into the gun housing. High voltage devices, on the other hand, typically do not fit through port plug openings, requiring insertion through one end of a separate arming sub or a special sub, for example, making the arming operation more difficult and adds cost and preparation time at the job site.
A high-voltage device that fits through a port plug opening is needed to reduce cost, improve reliability and improve well-site safety and efficiency. Added safety is afforded by a feature that only allows electrical power to initiate the device by sending a prescribed activation signal.